298 GENERAL ORNITHOLOGY part ii 



the contiguous air-receptacles, and are affected equally and regularly 

 by every motion of the sternum and ribs. A third use, and per- 

 haps the one which is most closely related to the peculiar exigencies 

 of the bird, is that of rendering the whole body specifically lighter ; 

 this must necessarily follow from the desiccation of the marrow and 

 other fluids in those spaces which are occupied by the air-cells, and 

 by the rarefaction of the contained air from the heat of the body. . . . 

 A fourth use of the air-receptacles relates to the mechanical assistance 

 which they afford to the muscles of the wings. This was suggested 

 by observing that an inflation of the air-cells in the gigantic crane 

 {Gicon'm arcjala) was followed by an extension of the wings, as the 

 air found its Avay along the brachial and antibrachial cells. In large 

 birds, therefore, which, like the argala [or like the wood-ibis, Tantalus 

 loculator], hover with a sailing motion for a long -continued period 

 in the upper regions of the air, the muscular exertion of keeping 

 the wings outstretched will be lessened by the tendency of the 

 distended air-cells to maintain that condition. It is not meant to 

 advance this as other than a secondary and probably partial service 

 of the air-cells. In the same light may be regarded the use as- 

 signed to them by Hunter, of contributing to sustain the song of 

 birds and to impart to it tone and strength. It is no argument 

 against this function that the air-cells exist in birds Avhich are not 

 provided with the mechanism necessary to produce tuneful notes ; 

 smce it was not pretended that this was the exclusive and only 

 oflice of the air-cells." (Owen, Jnat. Vert., ii. 1866, p. 216.) 



Though nothing like them exists in mammals, it must not be 

 inferred that these air-pouches are unique in birds. The general 

 pulmonary mechanism is reptile-like, and the ornithic development is 

 simply a logical extreme of arrangements found in reptiles and lower 

 vertebrates, — even to the swim-bladder of a fish, which is morpho- 

 logically and homologically pulmonary, though fishes' gills are 

 functionally, and therefore analogicall}', their lungs, i.e. their 

 respiratory apparatus. 



The Trachea (Gr. Tpa^da, tracheia, rough) or " asper-artery " 

 answers perfectly to its English name, windpipe. It is the tube 

 which conveys air to and from the lungs (Fig. 101,^, to q). It 

 commences at the root of the tongue by a chink in the floor of the 

 mouth (Fig. 101, ^, c), runs down the neck in front between the 

 gullet and the skin, and ends below by forking into right and left 

 bronchus (Fig. 101, \ r, r). It is composed of a series of very 

 numerous gristly or bony rings connected together by elastic 

 membrane. Lengthening and shortening, effected by muscles to be 

 presently noted, is permitted by a very ingenious and interesting 

 construction of these rings, which will be clearly understood with 

 the help of the figures (96, a, b, 97, ^, -,) borrowed from Macgillivray's 



